With the increase in the number of input/output leads extending from electronic devices, such as integrated circuits, ball grid array (BGA) packages have been developed. A BGA package is a type of packaged electronic device in which at least one electronic device, such as an integrated circuit chip, is mounted to a substrate and an electrical connection to an electrically conductive material not part of the packaged electronic device, such as a printed circuit board (PCB), is made by an array of solder balls located on a surface of the substrate.
Current PCB technology used to manufacture BGA packages produce conductive trace lines having widths in the order of 3 mils or more. This trace line width limitation limits the number of input/output leads for each layer of conductive material attached to the BGA substrate. Thus, electronic devices requiring more than 100 input/output leads require multiple layers of conductive material wherein each layer must be electrically coupled together. This, in turn, requires creating through holes and/or vias to interconnect each conductive layer. For examples of BGA packages having multiple conductive layers and either through holes or vias, see U.S. Pat. No. 5,355,283.
Through hole drilling and/or via creation is one of the most expensive PCB operations. Each layer of interconnection increases the cost to the PCB. In addition, circuits which must travel across one layer, down a through hole, and back across another layer are much poorer in electrical performance than circuits which can travel directly across a single layer. Thin film circuitization processes such as vacuum metallization have been used to produce conductive traces having widths less than that which can be achieved using electroplating processes.
One of the keys to thin film circuitization technology is to deposit the conductive material on the substrate by evaporation or sputtering. Sputtering or evaporation can deposit very thin, uniform conductive material layers which, when used in combination with thin resists, can be defined to achieve traces having widths in the order of 1 mil. The limitation of sputtering and evaporation is that they are very hot processes. The substrate upon which the conductive material is deposited often reaches temperatures of 500 degrees Celsius. The substrate for thin film circuitization, therefore, is usually a high temperature material such as ceramic. Thin films of polyimide have also been used in combination with sputtered metal films and subtractive circuitization to produce multiple layers of thin film circuitry.
What is needed is a BGA package with a single layer of circuitry, such as conductive traces, wire bond pads, and solder ball pads having trace lines less than 1 mil in width so as to accommodate an electronic device having more than 100 input/output leads. Such a ball grid array package can be formed by using thin film circuitization processes such as vacuum metallization.